The present invention relates generally to multilayer circuit boards and, more particularly, to a technique for reducing the number of layers in a multilayer circuit board.
The making of electrical connections between electronic components has long been accomplished using printed circuit boards. The first such circuit boards had only a single signal layer on a top surface thereof for routing electrical signals between electronic components mounted thereon. These single signal layer circuit boards have severe limitations with regard to the number of electrical signals that can be routed between electronic components mounted on the same circuit board.
That is, the number of electrical signals that can be routed between electronic components mounted on a single signal layer circuit board is limited by the amount of area on the single signal layer.
The area limitations associated with single signal layer circuit boards led to the development of multilayer printed circuit boards. Such multilayer printed circuit boards may be either single or double-sided and may have multiple signal layers on the surface of and buried within the multilayer printed circuit boards. Thus, such multilayer printed circuit boards have allowed a large increase in the number of electrical signals that may be routed between electronic components mounted on the same circuit board.
The use of multilayer printed circuit boards has been particularly beneficial when using electronic components having high density packages. That is, electronic components having high density packages generally require multiple layers of a multilayer printed circuit board to make electrical connections with other electronic components mounted on the same circuit board.
In fact, the density of electronic component packages typically dictate the number of layers that must be provided by the multilayer printed circuit board upon which the electronic component is mounted. While the number of layers that may be provided by a multilayer printed circuit board is theoretically unlimited, problems occur when the number of layers in a multilayer printed circuit board exceeds a reasonable number, particularly when trying to route high speed electrical signals between electronic components. For example, when making electrical connections between different layers in multilayer printed circuit boards, electrically conductive vias are generally used. While these electrically conductive vias allow direct vertical electrical connections to be made between different layers within a multilayer printed circuit board, there are intrinsic parasitics associated with these electrically conductive vias that can adversely affect the performance of signals propagating therethrough. That is, these electrically conductive vias have intrinsic parasitic resistance, capacitance, and inductance, which can adversely affect signals propagating along each electrically conductive via. In addition, these intrinsic parasitics can also have an adverse effect on the manufacturability of a printed circuit board and thus the cost thereof. Because of their adverse affect on signal performance, these intrinsic parasitics can also limit the bandwidth of signals propagating along each electrically conductive via. These adverse affects only increase as the number of layers in a multilayer printed circuit board increase.
In view of the foregoing, it would be desirable to provide a technique for increasing the number of electrical connections that may be made between electronic components mounted on a multilayer printed circuit board without increasing the number of layers in the multilayer printed circuit board. More particularly, it would be desirable to provide a technique for reducing the number of layers in a multilayer circuit board in an efficient and cost effective manner.
According to the present invention, a technique for reducing the number of layers in a multilayer circuit board is provided. The multilayer circuit board has a plurality of electrically conductive signal layers for routing electrical signals to and from at least one electronic component mounted on a surface of the multilayer circuit board. In a preferred embodiment, the technique is realized by forming a first plurality of electrically conductive vias in the multilayer circuit board extending from the surface of the multilayer circuit board to a first of the plurality of electrically conductive signal layers, wherein the first plurality of electrically conductive vias are arranged so as to form a channel in a second of the plurality of electrically conductive signal layers beneath the first plurality of electrically conductive vias. The first plurality of electrically conductive vias beneficially electrically connect the at least one electronic component to the first of the plurality of electrically conductive signal layers. A first plurality of electrical signals are routed on the first of the plurality of electrically conductive signal layers. A second plurality of electrical signals are routed on the second of the plurality of electrically conductive signal layers in the channel formed in the second of the plurality of electrically conductive signal layers. The present invention technique is particularly beneficial when the at least one electronic component comprises an electronic component having a high density surface mount package such as, for example, a surface mount grid array package.
In accordance with other aspects of the present invention, wherein the first plurality of electrically conductive vias are arranged so as to form channels in other ones of the plurality of electrically conductive signal layers beneath the first plurality of electrically conductive vias, subsequent pluralities of electrical signals may be routed on the other ones of the plurality of electrically conductive signal layers in the channels formed in the other ones of the plurality of electrically conductive signal layers until all electrical signals to and from the at least one electronic component have been routed.
In accordance with further aspects of the present invention, the plurality of electrically conductive signal layers are typically separated by at least one dielectric layer. Also, at least some of the plurality of electrically conductive signal layers are typically separated by at least one electrically conductive power/ground plane layer. Further, the surface of the multilayer circuit board is typically primarily an electrically conductive power/ground plane layer.
In accordance with still further aspects of the present invention, a second plurality of electrically conductive vias may be formed in the multilayer circuit board extending from the surface of the multilayer circuit board to other ones of the plurality of electrically conductive signal layers. The second plurality of electrically conductive vias beneficially electrically connect the at least one electronic component to the other ones of the plurality of electrically conductive signal layers.
In accordance with still further aspects of the present invention, the first plurality of electrical signals to be routed on the first of the plurality of electrically conductive signal layers are preselected. The first plurality of electrical signals to be routed on the first of the plurality of electrically conductive signal layers may be preselected based upon a variety of signal characteristics. For example, the preselected electrical signals may be low speed electrical signals. Alternatively, the preselected electrical signals may be high speed electrical signals.